1. Field of the Invention
The present invention concerns the precorrection of nonlinearities in transmission systems and in particular the adaptive precorrection of nonlinearities in single-carrier or multicarrier transmission systems.
2. Description Of The Prior Art
As it is known, the final effect of the nonlinear distortion introduced by a power amplifier (HPA) is the in-band spectral energy variation and the appearance of out-of-band energies. The in-band energy variation causes a distortion in the transmitted signal (clustering: every point of the constellation scatters into a cloud of points; and warping: translation of the barycenters of the clusters), while the intermodulation products responsible for the out-of-band distortions cause interference with the adjacent channels. Both phenomena determine a reduction of the performances in terms of BER (Bit Error Rate) and of spectral efficiency.
An attenuation of these effects is obtained by using a high IBO (Input Back-Off) value, but this implies a reduction in terms of the HPA power efficiency. A more efficient solution consists in the use of predistortion techniques. In this way not only the effects due to the nonlinearities are reduced but also the power efficiency is increased, resulting in reduction of size and costs of the broadcast transmitters.
A predistortion device (PD) is a device able to generate a predistortion capable of compensating for the signal distortion introduced by the HPA. The final result of such a predistortion is the possibility of using the HPA at a higher power (lower IBO) at the same distortion or at the same power, thus providing a lower distortion.
Various predistortion techniques are known: "Feed-Forward", Predistortion" (with or without memory), and "Signal Predistortion" (at RF, IF, or BaseBand). The Feed-Forward technique is complex and fixed (not adaptive), requires a fine tuning, an additional power for the losses introduced by the main signal and an additional power for generating the signal distortion. The Data Predistortion technique only compensates for distortion at the sampling instants, does not eliminate the out-of-band distortions and is dependent on the type of modulation. Lastly, the Signal Predistortion generates a predistorted signal which should appear at the output of the HPA without any distortion. The combined effect of an ideal signal predistortion device with an HPA results in a linear function up to the saturation level (the so called ideal clipper). For this reason, the signal predistortion eliminates both in-band and out-of-band distortions until the signal exceeds the saturation level.
Theoretically, the signal predistortion can be applied at RF, at IF or in BaseBand. The RF approach is not desirable as it would theoretically require a specific circuit for each carrier and a perfect alignment at the various frequencies.
Apart from the type of implemented predistortion, a precorrection system is composed of a predistortion device (PD) and an estimator. The PD, by exploiting the information received from the estimator, performs a predistortion of the signal samples present at the input thereof. Conversely, the objective of the estimator is to estimate the phase and inverse amplitude characteristics of the amplifier. Further, it is preferable for the system to be instantaneous adaptive, having to follow possible variations of the amplifier characteristics due to thermal or aging phenomena or to the amplifier modules breakdown, i.e. in coincidence with each symbol from the source coming out from the PD, the predistorted symbol to be sent to the HPA must be obtained in real time.
It is necessary to take special care of the inverse amplitude characteristic estimation rather than the phase characteristic estimation. In particular, it would be desirable to directly estimate the inverse without having to perform further mathematical operations.
With reference to the baseband signal distortion, the precorrection system can be realized using two different schemes that differ each other as to the input data sent to the estimator: in one case (architecture "A", FIG. 1a) the estimator receives the signal samples which are present at the input (p.sub.n) of the PD and those, properly demodulated, coming out (R.sub.n) from the amplifier; in the other case (architecture "B", FIG. 1b) the estimator receives as inputs the signal samples, properly demodulated, coming into (r.sub.n) and coming out from (R.sub.n) the HPA and according to them, as in the previous case, carries out the estimation thereof
A system performing the baseband signal predistortion and according to the above architecture "A " is, in particular, known in the literature (from G. Santella, "Performance of Adaptive Predistorters in Presence of Orthogonal Multicarrier Modulation", International Conference on Telecommunications, pages 621 to 626, Melbourne, Australia 2 to 5 April, 1997). The content of such a published article is deemed as incorporated in the present description as a reference. The solution proposed by Santella implements a recursive procedure, dependent on both the starting conditions and on the step-size coefficients, converging to the stable point minimizing the mean square error. According to such a solution, a transformation is applied to the signal in such a way that, when it is amplified, the effects of the nonlinearities introduced by the HPA are compensated.
Among the drawbacks of such a solution are the fact that the algorithm is unstable for values of the step-size coefficients greater than a given threshold, the fact that the values of said coefficients do not significantly affect the rate of convergence but they affect the error oscillations and lastly the fact that in order for the convergence to be achieved, it will be necessary to scan the entire characteristic of the HPA at least 3/5 times.